1. Field of the Invention
The present invention relates to the field of optics, and more particularly, to a method and apparatus for measuring the polarization dependent loss of an optical component.
2. Related Art
When an optical signal is input to an optical component, an insertion loss occurs. A portion of the insertion loss varies as a function of the input polarization state and is known as polarization dependent loss (PDL). PDL causes signal degradation in optical transmission systems. Many optical components, such as isolators and couplers, exhibit PDL. It is desirable to accurately measure the PDL of an optical component to determine its suitability for use in an optical system.
Conventional techniques have allowed measurement of PDL to an accuracy of approximately 0.01 dB. However, in an optical transmission system which spans thousands of kilometers and includes many optical components located along the transmission path, a PDL of 0.01 dB per component can have a substantial adverse affect on signal quality. Thus, it is desirable to increase the accuracy with which PDL can be measured so that optical components may be more accurately screened to determine suitability for use in such an optical system.
Conventional PDL measurement techniques use a test set that includes a laser, a manual polarization controller, and a power meter. The polarization controller is used to manually adjust the polarization of an optical signal from the laser through a wide range of polarization states. The power meter is then used to monitor the output of the device under test (DUT) for maximum and minimum signal transmission. PDL measurement error can be attributed to three primary sources: (1) the stability of the laser, (2) the fiber bend loss associated with adjustments of the manual polarization controller, and (3) PDL of the power meter (detector).
Commonly owned, copending U.S. patent application Ser. No. 07/999,080, filed Dec. 31, 1992, and titled "Depolarizer," discloses a means for decreasing the PDL of the power meter. A depolarizer (an unpumped erbium doped fiber) is interposed between the DUT and the power meter. The depolarizer converts the polarized light into unpolarized amplified spontaneous emission of a longer wavelength. The unpolarized amplified spontaneous emission reduces the effect of the PDL of the power meter.
While the system disclosed in the '080 application has led to an improvement in PDL measurement techniques, a flaw remains. Current PDL measurement techniques are empirical. For example, the polarization controller is used to incrementally adjust the optical signal through a range of polarizations in search of maximum and minimum signal transmission through the DUT. The searches are normally done manually, although automatic searches involving measurements at virtually all possible states of polarization (SOPs) have been proposed. Regardless of whether the search is manual or automatic, it is likely that the actual (absolute) maximum and minimum points will be missed, because the search is done in discreet increments.
Manual searches also have the disadvantage of operator fatigue which results from the tedious process required to manually adjust an optical signal through a range of polarizations. Operator fatigue leads to measurement errors.
More sophisticated techniques involving gradient search procedures also appear feasible. The major drawback to these methods is that they are potentially time consuming. For example, the points on the Poincare sphere can represent the sampled SOPs to be used as a test input. If a sample is made every 10.degree. of longitude and latitude then roughly 600 measurements would form the sample set. This could take a considerable amount of time and is therefore not attractive as a manufacturing test procedure. In addition, for accurate PDL measurements one would have to be assured that the waveplates used to produce the sample SOP are without PDL.